Why are we asking, "How can we simulate the brain?"
Human intelligence is very general / broad in its scope. This is self-evident, and whatever AI ends up to be, we'd like it to be a general problem solver as well (cf. Simon and Newell). Taking liberal interpretations of your question...


  Why AI in a computer?


Computers, to the extent that we can frame problems in general as a solvable computational problem, are also general problem solvers. Wether this is actually the case (can you compute meaning or feels?) is up for debate (cf. computational functionalism, hyper-computation), but it is part of the artificial intelligence project to make a claim on this statement.


  Why do we think a computational framework brings us any closer to an understanding of cognition / consciousness?


Good question, and frankly there is no good answer to that asides from "its the best thing we got".

TL;DR "computational functionalism", a lot of the literature in psychology and philosophy seems to converge towards an understanding of cognition as "computational" (as in information processing: the V1 stream in the brain processes "early visual information") and functional (goal directed grounded on "meaning", ex: "i scratch itch because itchy", as opposed to "i am moving atoms"). 

However the two theories don't mesh together well (cf. Chinese Room Argument, and the many other arguments in a similar flavour) despite their independent successes in the theory of mind. Why this is the case nobody quite knows...


  Why not AI in something that isnt a computer?


I don't know, but to the extent that our understanding of the world is grounded in math, then it being in a computer is sufficient anyways.

Maybe there are other paradigms of understanding the world though. Fingers crossed 


  Why are we asking, “How can we simulate the brain?”


Because its the best tentative understanding we have of an "intelligent faculty", though it should be noted that various methods in machine learning don't seem to be directly inspired by biological implementation  (kNN, statistical methods, as opposed to neural nets)

Further reading: http://www.scaruffi.com/nature/mach01.html